Method for frequency acquisition of a mobile communications device

ABSTRACT

A method is provided for frequency acquisition, particularly for initial frequency acquisition, pursuant to a known synchronization sequence for synchronizing a mobile communications device having a local oscillator with previously known transmit frequencies of a base station that transmit in a known channel raster with defined frequency points within a band, wherein the method includes the steps of determining the inband power of the known synchronization sequence via a sensor by scanning a frequency band, performing coarse determination of the local power maximum of the inband power and, thus, of the received carrier frequency over the scanned frequency band, producing a presumed channel frequency at which the base station is transmitting on the basis of knowledge of the channel raster of the transmit frequencies of the base station, performing fine determination of the received carrier frequency by comparison with the known synchronization sequence, and correcting the frequency deviation of the local oscillator from the transmitted carrier frequency.

The present invention relates, generally, to a method for frequencyacquisition and, more particularly, to a method for initial frequencyacquisition via a known synchronization sequence for synchronizing amobile communications device having a local oscillator with thepreviously known transmit frequencies of a base station that transmit ina known channel raster with defined frequency points.

In mobile communications devices, a standard for mobile communicationmakes available a synchronization sequence known throughout the network.In addition, the transmit frequency of the base station is known with asufficiently high accuracy specified by the standardization bodies. Thisaccuracy is currently 0.05 ppm in the UMTS Standard. The channel rasterin which the base station transmits is known in this case a priori tothe mobile station. In the case of a UMTS mobile radio network, theraster includes channels at a spacing of 200 kHz at defined frequencypoints. By contrast, it is not known which channel is currently beingused by the base station, nor which frequency drift the oscillator ofthe mobile receiving station has. The manufacturing tolerances andtemperature spreads, as well as other possible external influencesresult in a frequency inaccuracy of the local oscillator which istypically +/−25 ppm. The frequency inaccuracy is mostly still above thisvalue when use is made of cost-effective local oscillators (LO). Such alarge inaccuracy is disadvantageous, since this leads to a complicatedand lengthy synchronization of the mobile part with the base station.This problem has so far been solved by calibrating the mobile parts ofthe communications device during fabrication, thus lowering thefrequency drift of the local oscillator to approximately +/−3 ppm.However, this is very time-consuming and cost-intensive.

Moreover, it has been proposed, for the purpose of initialautocalibration when first switching on the mobile part, given afrequency inaccuracy of, however, +/−25 ppm, to make use of an algorithmin the case of which a carrier, modulated with the aid of an SCH channel(synchronization channel) of the base station, is scanned by an SCHcorrelator accompanied by variation of the receive frequency. In thiscase, use has so far been made as SCH correlator only of a singleso-called matched filter, which is matched to the synchronizationfrequency of the primary SCH channel, which is denoted from now on asSCH channel, for short. After initial autocalibration has beenperformed, a frequency accuracy of +/−3 ppm can be assumed in thesynchronization operations following thereupon. Since the correlationbandwidth for the codes used is small according to specification—(on theorder of magnitude of 10-20 kHz) by comparison with the scanning range,whereas the scanned frequency band can include the complete UMTSbandwidth of 60 Mhz, this method is very time-consuming and costly. Stepsizes smaller than 14 kHz must be used in this case for correlation.This step size can be obtained from simulation results. The initialfrequency acquisition requires 6000 steps for a step width used of 10kHz, for example.

It is, therefore, an object of the present invention to make available amethod via which a frequency acquisition in the mobile part of a mobilecommunications device can be accelerated.

SUMMARY OF THE INVENTION

The basic idea of the present invention consists in determining theinband power via a sensor in parallel with the conventional SCHcorrelation which is used, as previously, as an integral constituent ofthe newly developed method. The presumed channel frequency at which thebase station is transmitting is deduced from the determination of thelocal power maximum over the entire scanned frequency band. The channelspacing, known a priori, of the possible discrete channel frequencies isused for this purpose in a supplementary way. The channel spacingbetween the frequency points is 200 kHz in the case of a UMTS mobileradio network. Moreover, the transmit frequency of the base station isknown with a sufficiently high accuracy, specified by thestandardization bodies, of +/−0.05 ppm. Given a maximum frequency of2.17 GHz, the result is a maximum uncertainty of approximately 20 Hzwith reference to the signal transmitted by the base station. This highaccuracy is achieved by GPS, or DVF77 signals. It is thereby possiblefor the channel frequencies emitted by the base station to be used asdifferential signals for tuning the local oscillator of the mobilestation. This is attended by the advantage that the synchronization isperformed substantially faster, which leads to a greatly reduced powerconsumption in the mobile part. As such, the standby times of the mobilepart can be significantly increased.

It is preferred to apply the method according to the present inventionupon the first startup and/or each further xth startup of the mobilecommunications device. Aging effects and temperature deviations of thecomponents that are used to generate frequency are thereby automaticallyrecalibrated. Since this takes place automatically during callup, noadditional outlay is caused. By contrast, calibration via thetemperature during fabrication would be time-consuming andcost-intensive. Moreover, the communications device can be matchedadaptively to the user's surroundings in the case of the methoddescribed. Again, this method leads to an increase in the productservice life owing to the recalibration rendered possible. Thedetermination of the dummy x is a function of a number of factors; forexample, the temperature fluctuations in the surroundings of theoscillator, and the age of the latter. The value can be 1, such that arecalibration is performed with each synchronization, or else, forexample, 2, 10, etc. The larger the value of x, the less is the powerconsumption, and longer standby times of the mobile part can beachieved.

It is preferred, furthermore, when the accurately determined frequencyof the local oscillator is adopted after the synchronization asreference by storing the determined hardware calibration parameters in amemory of the mobile communications device. It is possible as a resultto undertake a first calibration of the local oscillator in the RFsection of the mobile station. That is to say, the algorithm in thiscase automatically adopts the settings for calibrating the localoscillator which otherwise would have to be undertaken during theproduction process.

In particular, this method can be applied in the case of a UMTS mobileradio network, the determination of the inband power being performed bya power sensor, and the fine determination of the received carrierfrequency being performed by an SCH correlator.

One embodiment of the present invention provides for synchronizing themobile communications device with a first base station and carrying outthe callup to this base station, but a switch is made to a second basestation during the call. This is advantageous whenever it emerges duringa call that another base station is being received more strongly thanthe base station with which synchronization was originally carried out.Consequently, by comparison with the known method, a higher level ofreliability and of frequency accuracy is achieved than in the case of aconventional method in which only an SCH correlator is used. The reasonfor this is that the results are confirmed independently of one anothersimultaneously by the SCH correlator and the power sensor. As such, inthe case of a UMTS mobile radio network, each of the possible channelsof this network can be used for synchronization. For example, it ispossible to perform synchronization with channel 2 of a base station ofa first operating company; the actual callup taking place, however,subsequently on channel 5 of another base station of a second operatingcompany. This is possible because the UMTS base stations aretransmitting at approximately the same frequency, the difference betweenthem being only +/−1 ppm.

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the following DetailedDescription of the Invention and the Figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a schematic of the cycle of an exemplary embodimentaccording to the present invention.

FIG. 2 shows the frequency bands to be scanned for the individual stepsduring the method according to FIG. 1.

FIG. 3 shows a measuring instrument of a power sensor over a frequencyprofile.

FIG. 4 shows the measurement result of FIG. 3, combined with themeasurement result of a pSCH correlator over a frequency band.

DETAILED DESCRIPTION OF THE INVENTION

Specified in FIG. 1 are the individual groups of information that flowtogether in an evaluator 1 and from which the frequency acquisition canbe undertaken. Firstly, the a-priori knowledge 2 about the channelraster of the base station flows into the evaluation, as does theresult, measured with the aid of a power sensor 3, of the inband powerover a frequency band, as illustrated in FIG. 3. Also flowing in is theinformation that is obtained via a primary synchronization channel(pSCH) 4, as illustrated in FIG. 4. Signal packets 5 are transmittedslotwise in the pSCH 4 at a temporal spacing T_(s1). The signal packets5 illustrated in FIG. 1 have a double-peak signal 6 from a first basestation and a single-peak signal 7 from a second base station. Thereceived signal packets 5 are subjected to slotwise accumulation 9 via amatched filter 8. After the data thus obtained has been processed via anindicator 10 and a peak detector 11, the information obtained therefromis passed on to the evaluator 1. The latter then performs the accuratedetermination of the received frequency of the local oscillator in themobile station 16 of the mobile communications device.

The cycle of an accurate determination of the frequency drift of thelocal oscillator, and thus of the calibration of the latter with the aidof a UMTS mobile radio network, is shown in FIG. 2. The frequency bandcovered by UMTS is 60 MHz wide and has enumerated channels. The centercarrier frequency of the base station is approximately 2.1 GHz. Theinband power, which has a bandwidth of approximately 3.84 MHz, ismeasured in a first iteration step by the power sensor 3 for the purposeof coarse determination of the transmit frequency of the base station. Acoarse search is thereby placed ahead of the actual correlation. Inconjunction with the a-priori knowledge of the channel raster, theresults obtained fix the received channel frequencies, which stem fromdifferent base stations, as early as in the preselection from +/−200kHz, since this corresponds exactly to the channel raster. As such, thereceived channels are determined in the 200 kHz raster by the powersensor 3. The maximum determined by the power sensor 3 is atapproximately 10 MHz, this frequency already being reduced about thecenter carrier frequency of the base station. The step width can be 1MHz because of the a-priori knowledge of the channel raster, it beingpossible to determine the channel frequency up to +/−200 kHz; that is tosay, the power sensor 3 determines the received channels in the 200 kHzraster on which the base stations are transmitting. It is also possibleto use a step width of over 1 MHz; however, it is preferred to use thelesser step width of 1 MHz when several base stations are transmitting.

The fine adjustment of the frequency of the local oscillator isperformed via the pSCH correlator, which is specified according to theprescribed standard. The results of this correlator are used for thepurpose of correcting frequency deviations of the local oscillator thatexist because of manufacturing tolerances and temperature fluctuations.

On the basis of the channel raster thus found, it is possible to use asecond iteration step to achieve the fine tuning of the oscillator driftvia the pSCH oscillator with a substantially lower outlay. During thesecond iteration step, the received signal is compared with the pSCHsequence already known and uniform throughout the network. The pSCHcorrelator has a very narrow correlation bandwidth, or clear scanningresult, which is, however, of very high power and obtained here only inconjunction with very low frequency detuning. The correlation bandwidthis 10-20 kHz, particularly approximately 16 kHz, and this can beobtained from simulation results. Other values also can result for thecorrelation bandwidth if the synchronization codes change during thestandardization process.

Since the channel frequency is already determined up to +/−200 kHz, theexact frequency at which the base station is transmitting can bedetermined in few steps in a third iteration step with a step widthbelow the correlation bandwidth; for example, 10 kHz here.

The third iteration step is, however, unnecessary when the a-prioriknowledge about the channel raster is used, and the local oscillator(LO) of the mobile station has already been so well calibrated that ithas a remaining inaccuracy of only +/−3 ppm; this corresponding to 6kHz.

FIG. 4 shows a diagram in which, firstly, the inband power 12illustrated in FIG. 3 and its maximum 13 are shown. Secondly, the output14 of the pSCH correlator in the baseband is plotted below the inbandpower 12 of the power sensor 3. Here, as well, a correction wasundertaken by the carrier frequency of the base station of approximately2.1 GHz. The output 14 of the pSCH correlator has a correlation peak 15at 10 MHz. The actual natural frequency of the local oscillator in themobile station can be deduced from this result, since the channel rasterwith which the base station is transmitting is known a priori to themobile station. In this case, the carrier frequency of the base stationis precisely calibrated with an accuracy of 0.05 ppm. The deviation ofthe local oscillator of the mobile station 16 from the known referencefrequency is determined by comparing the position found for the measuredcorrelation peak 15 with the known channel frequency of the basestation. The data for autocalibration are thereby obtained. Themanufacturing tolerances in the natural frequency of the localoscillator of +/−25 ppm can be compensated to the extent that it ispossible to assume a maximum deviation of +1-3 ppm for futuresynchronization operations. Future synchronization processes are therebysubstantially accelerated.

If a first case of synchronization is involved, that is to say thedevice is being started up for the first time at the customer's, thereis a maximum deviation of 50 ppm from the reference frequency. As such,the pSCH correlation can be performed, for example, in 50 steps of stepwidth 1 ppm; that is to say, 2 kHz. After the first calibration has beenperformed, the maximum permissible deviation is then only 3 ppm. ThepSCH correlation then requires only six steps given an identical stepwidth of 2 kHz. As such, after initial synchronization has beenperformed, the automatic frequency acquisition described operates almost10 times more quickly.

During the initial calibration of the local oscillator in the RF sectionof the mobile part 16, the algorithm can undertake settings forcalibrating the local oscillator that otherwise would have had to beundertaken during the production process. Instead of undertaking thecalibration during the production process, this is done automaticallyduring the first startup of the device at the customer's. After thesynchronization, the accurately determined frequency of the localoscillator is taken over as reference by storing the hardwarecalibration parameters.

Although the present invention has been described with reference tospecific embodiments, those of skill in the art will recognize thatchanges may be made thereto without departing from the spirit and scopeof the present invention as set forth in the hereafter appended claims.

1. A method for frequency acquisition via a known synchronizationsequence for synchronizing a mobile communications device having a localoscillator with previously known transmit frequencies of a base stationthat transmit in a known channel raster with defined frequency pointswithin a band, the method comprising the steps of: determining an inbandpower of a received carrier frequency via a power sensor by scanning afrequency band; performing coarse determination of a local power maximumof the inband power at a coarse carrier frequency, and thus of thereceived carrier frequency over the scanned frequency band; producing apresumed channel frequency at which the base station is transmittingbased on the coarse carrier frequency and knowledge of the channelraster of the transmit frequencies of the base station, the presumedchannel frequency being different than the coarse carrier frequency; andsimultaneously performing fine determination of the received carrierfrequency using a synchronization channel, the synchronization channelhaving a correlation peak at a frequency that is different than thepresumed channel frequency and different than the coarse carrierfrequency.
 2. A method for frequency acquisition as claimed in claim 1,wherein the method is applied upon at least one of a first startup andeach further startup of the mobile communications device.
 3. A methodfor frequency acquisition as claimed in claim 1, the method furthercomprising the step of adopting an accurately determined frequency ofthe local oscillator as reference by storing the frequency in a memoryof the mobile communications device.
 4. A method for frequencyacquisition as claimed in claim 1, wherein the method is applied in aUniversal Mobile Telecommunications System (UMTS) mobile radio network,with the determination of the inband power being performed by a powersensor and the fine determination of the received carrier frequencybeing performed by an Synchronization Channel (SCH) correlator.
 5. Amethod for frequency acquisition as claimed in claim 1, wherein themobile communications device carries out a callup to the base stationand a switch is made to a second base station during the call.